Involvement of the brain cholinergic system in the rapid development of tolerance to glucose-induced analgesia

In this study, male Sprague-Dawley rats (150-230 g), maintained under controlled lighting and temperature conditions, were used. In one experiment, glucose administration (10 g/kg) was found to be associated with profound analgesia which could be blocked by prior administration of atropine (0.5 mg/kg). In another experiment, when two doses of glucose were given at 24 hr interval, the second injection of glucose was associated with tolerance to glucose-induced analgesia. In an attempt to correlate changes in the cholinergic enzymes with glucose-induced analgesia, choline acetyltransferase and acetylcholinesterase activities were determined in the cerebral cortex, bulbus olfactorius, midbrain, hypothalamus, hippocampus, cerebellum, pons and medulla oblongata in control rats and rats treated with a single dose of glucose (10 g/kg) or two doses of glucose. The second administration of glucose was accompanied with tolerance in the level of acetylcholinesterase in the bulbus olfactorius, midbrain, cerebral cortex, cerebellum and pons. There were no significant changes in choline acetyltransferase activities between the groups studied. The results obtained indicate that the cholinergic system may be involved in glucose-induced analgesia and that the rapid development of tolerance to glucose-induced analgesia is associated with the tolerance in the response of brain acetylcholinesterase activity.